First
synthetic gecko
adhesive which cleans itself during use, as the natural gecko does.
After contamination by microspheres, the microfiber array loses all
adhesion strength. After repeated contacts with clean glass, the
microspheres are shed, and the fibers recover 30% of their original
adhesion. The fibers have a non-adhesive default state, which
encourages particle removal during contact.Contact Self-Cleaning of Synthetic Gecko
Adhesive, Langmuir 2008

Before and after repeated contacts.

Overview of Gecko Adhesion Project

Geckos
have the remarkable ability to run at any orientation
on just
about any smooth or rough, wet or dry, clean or dirty surface. The
basis for geckos' adhesive properties is in the millions of
micron-scale setae on each toe of the gecko form a self-cleaning dry
adhesive. The tip of each seta consists of 100 to 1000 spatulae only
100 nanometers in diameter. Our interdisciplinary team of biologists
and engineers has been working since 1998 developing models for how the
natural
nanostructures function in a hierachical combination of spatulae,
spatular stalks, setal stalks, setal arrays, and toe mechanics, and
developing nanofabrication processes which allow large arrays
of hair patches to be economically fabricated.

Keywords: synthetic
gecko adhesion, gecko adhesive, gecko tape

Synthetic Gecko Nano Hair Properties

Using
insights from biology, we develop mechanical models
for gecko hair adhesion and then design, fabricate, and test micro and
nanofibrillar structures. We aim to achieve the seven benchmark
functional
properties of the gecko adhesive system identified by Autumn (MRS
Bulletin 2007):

The
low detachment force, self-cleaning, and non-sticky default state
suggest hard polymers, rather than the soft polymers
typically
used in pressure sensitive adhesives.
As reported in 2002, [Sitti and Fearing 2002]
and Autumn et al. [2002]
we have made synthetic spatulae, which have shown adhesion
similar to natural spatulae in the range of 100-300 nN.
These patches of bumps lacked the setal stalks, and achieved
adhesion forces on the order of a few milliNewtons on an
area of a square centimeter.
In 2003, we fabricated high density arrays of spatular stalks
[Campolo et al. 2003] which showed adhesion in shear
on the order of 0.5 Newton per sq. cm. In 2006, we demonstrated a novel
high friction array of 0.6 micron fibers
which showed shear resistance of 4 Newton per sq. cm. with only 0.8
Newton per sq. cm. of normal load [Majidi et al. PRL 2006]. In 2007, we showed how
the
polypropylene fiber arrays can provide shear force without a normal
load being present [Schubert et al. JAST
2007]. In 2008, we made easy-attach easy-release hard polymer gecko
adhesives which have a non-adhesive default state [Lee et al. JRSI 2008], and can self-clean
during contact [Lee and Fearing Langmuir
2008].
The final goal is to build arrays incorporating the necessary
geometrical features which have the same adhesion as geckos to rough
and smooth surfaces.

Interesting Facts about Gecko
Adhesion

Gecko toes are not ``sticky'' like tape.

If you touch a gecko toe it feels soft and smooth, and not sticky at
all.
If you pressed a gecko toe onto a hard surface it would not stick.
The toe will only adhere when the microfibers (setae) are engaged, by
dragging
or sliding the toe parallel to the surface. (If toes were sticky like
tape, it would be difficult for a gecko to walk
or run, as it would be too hard to pull its feet up.)

more
gecko facts ...There are many
groups starting to work on ``gecko-inspired''
adhesive materials. A summary of the main results are given in synthetic
gecko adhesive
comparison .
It is important to note that the natural gecko adhesive is
characterized by very low preloads and peel
strength, which is needed for climbing. (Much of the synthetic work is
aimed at achieving different characteristics
than the natural gecko adhesive, such as large normal forces.)
A brief synopsis of the history gecko-inspired synthetic adhesives
is here.

First
synthetic gecko
adhesive which cleans itself during use, as the natural gecko does.
After contamination by microspheres, the microfiber array loses all
adhesion strength. After repeated contacts with clean glass, the
microspheres are shed, and the fibers recover 30% of their original
adhesion.Contact
Self-Cleaning of Synthetic Gecko Adhesive, Langmuir 2008

Analysis of fiber adhesion in side contact.
[Majidi, Groff, Fearing J. Appl. Phys 2005] shows that
sufficiently long fibers, e.g. carbon nanotubes can stably make side
contact. This side contact can give 10-20 times greater adhesion force
than a hemispherical tip contact.

Using a casting process
in a template, polyurethane hairs 200 nm diameter and 60 micron long
were fabricated. Due to clumping, adhesion force was limited.

Synthetic spatula array (2002)

Synthetic spatula array
from nano-indenting and casting.
Approximately 200-300 nN adhesion force was measured per spatula. Total
area of array was less than 100x100 sq. um. [Sitti
and Fearing IEEE Nano 2002]

Single spatula were constructed from silicone rubber (E ~ 0.5 MPa) and
polyester (E ~ 1000 MPa) by using nano-indentation and casting.
Using atomic force microscope, 290 nN pulloff force was measured from a
single polyester spatula with tip radius of 350 nN, and 180 nN for
silicone rubber. The similarity of pulloff forces supports the
hypothesis
of material independence for gecko adhesives.
[Autumn
et al. PNAS 2002]

Sponsored by NSF
``Electrically-Controlled Nanofibrillar Surfaces for Cleaning and
Adhesion''
(2009-2012)
Sponsored by NSF
NIRT:
``Biologically Inspired Synthetic Gecko Adhesives'' (2003-2009).
NSF Disclaimer:
``This material is based upon work supported by the National Science
Foundation
under Grant No. EEC-0304730. Any opinions, findings and conclusions or
recommendations expressed in this material are those of the author(s)
and do not necessarily reflect the views of the National Science
Foundation
(NSF).'' (previous sponsorship by DARPA (1999-2003) "Biomimetic
Climbers" .)